Skip first run experience locally and on azure pipelines.

- This would cause our functional tests to time out and occasionally crash due to dotnet first run experience sentinels being locked.

aspnet/AspNetCore-Internaldotnet/aspnetcore-tooling#1859
\n\nCommit migrated from 2668a0e8a9

src/Razor/Microsoft.NET.Sdk.Razor/test/IntegrationTests/MSBuildProcessManager.cs

@@ -41,6 +41,10 @@ namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
             {
                 processStartInfo.FileName = DotNetMuxer.MuxerPathOrDefault();
                 processStartInfo.Arguments = $"msbuild {arguments}";
+
+                // Suppresses the 'Welcome to .NET Core!' output that times out tests and causes locked file issues.
+                // When using dotnet we're not guarunteed to run in an environment where the dotnet.exe has had its first run experience already invoked.
+                processStartInfo.EnvironmentVariables["DOTNET_SKIP_FIRST_TIME_EXPERIENCE"] = "true";
             }
 
             var processResult = await RunProcessCoreAsync(processStartInfo, timeout);